Energy value of food

The energy value of foods is currently expressed in Kcalories (Kcal) and in Kjoules (Kj), the conversion factor being 4.184 to obtain Kjoules from Kcalories. This dual system emerged from a recommendation of the International Union of Nutritional Sciences to express the energy content of foods also in joules, since they represent a more scientifically correct unit to describe biological work, even though the use of calories is still predominant.

The chief food sources of energy to the human body are fat, carbohydrate, and protein. Fats and carbohydrates contain carbon and hydrogen which can be oxidized to their end products, CO2 and H2O, both in the bomb calorimeter and in the body. In addition, protein contains nitrogen. This nitrogen together with some carbon and hydrogen leaves the body chiefly in the form of urea.

The energy yielding food factors are:
-Carbohydrates: banana, breadfruit, jackfruit, plantain, dates, prunes, raisin
-Proteins and amino acids: nuts, dried apricot, fig
-Fats: avocado, olive, nuts

The energy value of foods can be expressed in terms of kilo calories (KCal) or megajoules (MJ).
*Kilo Calorie: One kilo calorie is the quantity of heat required to to raise the temperature of 1kg of water through 10C. It is one thousand times the small calorie used in physics measurements.
*Mega Joule : One kilo calorie equals 4.186 kilo joules. Hence thousand kilo calorie equals 4.186×103kilo joules or 4.186 mega joules.

Most of the fundamental work on the energy value of foods was carried out by the pioneer scientists Rubner (in Germany) and his pupil Atwater (in USA) at the end of the nineteenth century. Rubner measured the heats of combustion of a number of different proteins, fats, and carbohydrates in a bomb calorimeter and also studied the heat of combustion of urine passed by a dog, a man, a boy, and a baby.
Energy value of food